Near atmospheric pressure treatment of polymers using helium discharges

ABSTRACT

The present invention is a method and apparatus for treating a polyester support such as polyethylene napthalate or polyethylene terephthalate. The treatment is carried out at near atmospheric pressure in a gas of helium and nitrogen or oxygen. The treatment uses anodized aluminum electrodes and an atmospheric glow discharge results when the electrodes are connected to an RF generator and spaced about 2 mm apart. The process and apparatus improve adhesion of subsequently coated emulsions on the polyester support.

FIELD OF THE INVENTION

The present invention is a novel process for treating photographicsupport with electrical discharges at atmospheric pressure to promoteadhesion of subsequent coated layers.

BACKGROUND OF THE INVENTION

Corona discharges are used widely to promote adhesion between variousmaterials. In manufacturing photographic products there is a large bodyof literature describing various applications of coronas to make aqueousand non-aqueous coatings adhere to various substrate materials. Almostall of these coronas are produced by applying a high voltage(approximately 5-10 kV), relatively high frequency (10 kHz) signal toelectrodes in an ambient atmosphere at atmospheric pressure. See forexample, U.S. Pat. Nos. 4,241,169; 4,701,403; 4,087,575; 4,429,032;4,363,872; 4,229,523; 4,394,442; 3,411,908; 3,531,314; 3,582,339;3,607,345; 3,630,742; 3,860,427; 3,874,877; 3,888,753; 4,055,685;4,518,681; 5,004,669; French Patent 7613034 and European PatentApplication No. 92303556.2. However, there are limitations to theusefulness of corona treatments. Corona treatments produce locallyenergetic discharges, commonly known as streamers, and these streamersmay cause a non-uniform level of treatment. They may also be related toan inhomogeneous loss of red speed in photographic emulsions whichproduces a mottle defect. Furthermore, corona treatments appear to beeffective at promoting adhesions of various coatings to polyethylene butare relatively ineffective at promoting the adhesion of layers tovarious polyester supports such as polyethylene terephthalate (PET) andpolyethylene naphthalate (PEN).

A more controllable and effective way of preparing polymers for coatingis with a low pressure glow discharge treatment. Glow discharges are, bynature, very diffuse and homogeneous, producing a more uniformtreatment. Moreover, by controlling the gas it is possible to improvethe adhesion of photographic layers to materials such as polyesters aswell as polyethylene. See for example, U.S. Pat. Nos. 4,993,267;3,837,886 and 4,451,497. A major disadvantage in glow dischargetreatments done at reduced pressures is the problem of maintaining a lowpressure at the treatment station. It is necessary to use either a batchprocess, in which the support is loaded into a treatment chamber and theair is removed, or an in-line process which requires that the supportpass through a differential pressure region. In the first case, thesupport must go through an additional off-line step before coatings areapplied. This is unattractive from a product flow perspective andrequires additional capital. The second choice is difficult andexpensive to implement because of the very tight tolerances needed tomaintain the pressure differentials in the transport path. This requiresexpensive and complicated hardware and pumps. The closer to atmosphericpressure that the treatments can be done, the simpler and less costlythe process becomes.

It is known that under certain conditions, stable diffuse glowdischarges can be produced at atmospheric pressures. Articles thatdiscuss stable glow discharges are as follows: S. Kanazwa, M. Kogoma, T.Moriwaki and S. Okazaki, J. Phys. D: Appl. Phys 21 (1988), pp. 838-840;S. Kanazwa, M. Kogoma, S. Okazaki and T. Moriwaki, Nuclear Instrumentsand Methods in Physics Research, B37/38 (1989) pp. 842-845; T. Yokoyama,M. Kogoma, S. Kanazawa, T. Moriwaki and S. Okazaki, J. Phys. D: Appl.Phys. 23 (1990) pp. 374-377; T. Yokoyama, M. Kogoma, T. Moriwaki and So.Okazaki, J. Phys. D: Appl. Phys. 23 (1990) pp. 1125-1128 and A. Nagata,S. Takehiro, H. Sumi, M. Kogoma, S. Okazaki, and Y. Horikie, Proc. Jpn.Symp. Plasma Chem 2 (1989) pp. 109-112. Work in this area has beenlimited and directed primarily at etching of photoresist and depositionof materials. These articles indicate that a reliable method ofproducing diffuse glow discharges at atmospheric pressure is to usehelium as the discharge gas. The work reported in the literature hasbeen reproduced and found to be reliable. It has also been found thatvery small amounts of reactive gases, such as a few percent nitrogen oroxygen, will extinguish an atmospheric helium discharge. However, wehave also found that by using trace amounts of reactive gases, stabledischarges can be produced at very close to atmospheric pressure whichare also able to dramatically improve the adhesion of photographicemulsions to difficult to coat materials such as polyethylene, PET andPEN.

The present invention allows one to treat polymeric surfaces with astable atmospheric glow discharge so that adhesion of photographicemulsions is improved between the photographic emulsions and thepolymeric materials.

SUMMARY OF THE INVENTION

The present invention is a method of treating a polymeric support. Themethod includes providing a first electrode having a first surface, thefirst electrode having a plurality of spaced apart holes adjoining thefirst surface, the first surface being insulated. A second electrodehaving a second surface is positioned in a spaced apart relationshipfrom the first surface of the first electrode. Gas is pumped through theplurality of holes at a pressure greater than or equal to atmosphericpressure. The gas comprises helium, oxygen and nitrogen. A power supplyis coupled to the first electrode, the power supply has a frequency ofbetween 10 kH to about 50 MHz. A web is positioned between the firstsurface of the first electrode and the second surface of the secondelectrode wherein the polymeric web is subjected to atmospheric glowdischarge to improve the adhesive properties.

The present invention also includes a device adapted for use in anatmospheric glow discharge chamber which includes an electrode having afirst surface, the first surface including a plurality of holesterminating at the first surface, the first surface of the electrodebeing insulated. A gas is provided to the plurality of holes and a powersupply is coupled to the electrode.

The present invention provides the advantage of improving the adhesiveproperties of a polyester substrate using glow discharge device thatoperates at atmospheric pressures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a prior art device used to obtain anatmospheric glow discharge.

FIG. 2 shows an electrode configuration of the present invention for thecontinuous treatment of a moving web.

For a better understanding of the present invention together withobjects, advantages and capabilities thereof, reference is made to thefollowing detailed description and appended claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a prior art set-up used to obtain a near atmosphericpressure glow discharge. Two solid square aluminum electrodes 10 and 11,one of which was anodized (electrode 10), were used to treat fullyoriented PET and oriented annealed PEN in helium and in mixtures ofhelium and oxygen or nitrogen. The electrodes 10 and 11 were 7.5 by 7.5cm and were 2 mm apart. They were powered by an RF generator 12operating at 13.56 MHz. With a mixture of 0.1% N₂ in He by volume, astable discharge was possible at 600 Torr or below. Greaterconcentrations of reactive gas (either N₂ or O₂) further lowered theavailable operating pressures for stable discharges.

FIG. 2 shows a sectional view of the atmospheric glow dischargeapparatus used in the present invention. Electrode 20 includes a seriesof apertures 23 through which the gas is supplied through inlet 24. Thedimensions of electrode 20 are 12.1 cm by 13.3 cm. Electrode 20 has 333apertures 23 each having a 1 mm diameter. The apertures aresymmetrically distributed on the electrode 20. Surprisingly, it has beenfound that a stable glow discharge at atmospheric pressure with higherpercentages of reactive gas species, most notably N₂ and O₂, is possibleusing the electrode 20 shown in FIG. 2. This allows for a faster andmore complete treatment of the polyester substrate. The perforatedelectrode configuration shown in FIG. 2 can be operated in ambient airwith a mixture of 8% N₂ in He being supplied through the apertures 23.Using the solid electrodes of FIG. 1 a stable discharge was not possibleusing the same gas mixture.

In order to demonstrate the improved adhesion properties of PET and PENavailable from the apparatus and method of the present inventioncomparative adhesion tests were run between the solid electrodes of theprior art and the perforated electrode of the present invention.

After treatment, the substrates (PEN and PET) were coated with a blackand white film emulsion. After the emulsion was set and dried a seriesof adhesion tape tests were run to test the adhesive properties of thetreated PET and PEN.

Initially, the prior art electrodes of FIG. 1 were placed inside avacuum vessel and the pressure was reduced to about 50 mTorr. Thetreatment gas was then introduced into the chamber. The total pressurefor all the treatments was 0.79 atmospheres (600 torr). The treatmenttime was 120 seconds in all cases.

The table below shows the results of comparative adhesion tests done ona black and white emulsion coated on either polyethylene terephthalate(PET) or polyethylene naphthalate (PEN) after various treatments. Toquantify the adhesion, three types of 3M Scotch™ brand tapes withincreasing adhesive strengths are used. Test A uses Type 810, Test Buses Type 610 and Test C uses Type 396. The amount of emulsion removedwas evaluated as a percentage of the area tested. Without any treatment,the emulsions fell off of the support.

                  TABLE 1                                                         ______________________________________                                                                Pressure                                              Run                     Pct. Gas                                                                             Tape  Tape  Tape                               No.   Support  Gas      in He  Test A                                                                              Test B                                                                              Test C                             ______________________________________                                        1     PET      Pure He  0      95                                             2     PET      O.sub.2  1      50    10    80                                 3     PET      N.sub.2    0.1  10    95                                       4     PEN      Pure He  0      10    10    95                                 5     PEN      O.sub.2  1       0     0     0                                 6     PEN      N.sub.2    0.1  10     0     0                                 ______________________________________                                    

We see from this data that, while pure He discharges can produce someimprovement in adhesion, particularly on PEN, addition of the reactivegas vastly improves the results for either material. However, operationbelow atmospheric pressure is disadvantageous from a manufacturing pointof view because of the complexity of getting substrate material into andout of the low pressure zone.

The tape tests used to evaluate the effectiveness of the atmosphericglow discharge treatments on the PET and PEN using the present inventionincluded application of a piece of Scotch™ brand adhesive tape to thedried emulsion coated substrate, and evaluating the results of peelingthe tape from the sample. Five tests were used, each one successivelymore aggressive than the previous test. This sequence was achieved bychanging the tape type, tape width, type of scoring tool, type ofscoring, and tape peeling speed.

A black and white emulsion was coated on the PET and PEN substratestreated by the present invention in a 4 inch wide stripe down the 18inch length of a 6 inch wide web sample. Either a high speed steel (HSS)tool bit or a dissection scalpel is used to scribe a scratch 1/2 inch infrom each edge of the emulsion stripe. When patterns ("H" or "#") areused, the HSS bit or scalpel is used to form the pattern in the emulsionsurface. A piece of the specific tape is then hand applied and pressedonto the prepared area. The length of the leader, or pull tab, is testspecific to further control the peel force.

The tapes used include 810 (1/2 inch width), 610 (1 inch width), and 396(3/4 inch width). One of the tool bits may be used to slice the emulsionat the edge of the tape to concentrate the peel stresses to the areaunder the tape. Or, the peel forces can be spread out by not scribingthe edges. In each case, the tape is then peeled such that the peelangle is 90 degrees between the tape and substrate. The speed of thepeeling motion is another factor which affects the aggressiveness of theparticular test. Two of the tests utilize multiple (3) peels to increasethe aggressiveness.

A summary of the tests, in order of increasing aggressiveness, follows:

                  TABLE 2                                                         ______________________________________                                        Tape                              Edge       # of                             Test Tool     Pattern Tape Leader Slice                                                                              Speed Peels                            ______________________________________                                        D    Scalpel  none    810    0.25"                                                                              No   Slow  1                                E    Scalpel  none    810    0.25"                                                                              Yes  Fast  1                                F    HHS Bit  H       810  4"     Yes  Fast  3                                G    Scalpel  #       610  4"     Yes  Fast  3                                H    Scalpel  #       396  2"     Yes  Fast  1                                ______________________________________                                    

Referring to Table 1, Test E is similar to Test A, Test G is similar toTest B and Test H is similar to Test C. However, due to the peel speedsand scoring of the emulsion, Tests E, G and H are more aggressive thanTests A, B and C, respectively.

The amount of emulsion removed by the tape is recorded for each test asa percentage of the original bounded area under the tape.

A series of treatments was done on a small web system using theperforated electrode configuration as shown in FIG. 2. The tests wererun at atmospheric pressure. The powered electrode 20 is anodizedaluminum and is curved to conform to the transport drum 30 over whichthe web 31 passes. The electrode 20 is coupled to power supply 12. Onceagain, a black and white emulsion was coated onto the treated supports.

The table below shows the results of the tests for various treatments ofPEN and PET. The numbers in the test columns show the amount of emulsionremoved by each procedure. Without any treatment, the emulsions lift offof the supports by hand.

                                      TABLE 3                                     __________________________________________________________________________    Run        Flow Pct.                                                                           Treatmt                                                                            Test                                                                             Test                                                                              Test                                                                             Test                                                                              Test                                      No.                                                                              Support                                                                            Gas                                                                              Gas in He                                                                           Time D  E   F  G   H                                         __________________________________________________________________________    1  PET  N.sub.2                                                                          1.7   48 sec                                                                              0  0   40                                                                              80   75                                       2  PET  N.sub.2                                                                          1.7   480 sec                                                                             0  0  100                                              3  PET  N.sub.2                                                                           0.34 48 sec                                                                             100                                                                              97  100                                                                              99  100                                       4  PET  N.sub.2                                                                           0.34 480 sec                                                                            100                                                                              100 100                                                                              100 100                                       5  PEN  O.sub.2                                                                          0.5   48 sec                                                                              0 20   85                                                                              85   95                                       __________________________________________________________________________

Run numbers 1-4 demonstrate that for atmospheric pressure He/N₂treatments of PET, regardless of the treatment time, higher percentagesof reactive gas give improved adhesion. Run number 5 demonstrates thatgood results are possible with PEN support using an O₂ discharge. Thereis no evidence in these discharges of the streamers often associatedwith corona discharges, which can lead to non-uniform treatments. Incomparing the results of Table 3 with the results of Table 1, it is alsonoted that a stable discharge using 1.7% N₂ in He at atmosphericpressure could not be achieved using the prior art electrodes.

While there has been shown and described what are at present consideredthe preferred embodiments of the present invention, it will be obviousto those skilled in the art the various changes, alterations andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

What is claimed is:
 1. A device adapted for use in an atmospheric glowdischarge chamber comprising:an electrode having a first surface, theelectrode including a plurality of holes terminating at the firstsurface, the first surface of the electrode being insulated; a groundedtransport drum for carrying a web to be transported between the drum andthe electrode; means for providing gas to the plurality of holes; apower supply coupled to the electrode.
 2. The device according to claim1 wherein the electrode comprises aluminum and the aluminum on the firstsurface of said electrode is insulated by anodizing.